Functions and properties of CANopen devices (field devices), for example, are largely described using objects and are managed in an object dictionary. The object dictionary is represented in an electronic form in so-called EDS files (Electronic Data Sheet). EDS files are an inherent part of standardization in the CANopen environment (CiA DS306 and CiA DS311) and have a fundamental importance when the configuration in CANopen networks is involved. The file format specified by CiA ensures that the device descriptions of tools (for example ProCANopen) of different software companies can be read and processed. An EDS file (electronic data sheet) is therefore an example for a device description file which is permanently stored as firmware on a field device and contains all information concerning the device-specific parameters and operating modes of the device. Using the device description file, the device is properly configured or put into operation.
It is basically possible to make a distinction between field devices having a permanent configuration and field devices having an adjustable configuration. In a field device having a permanent configuration, it is clearly defined which process data (for example digital inputs or current speeds) are transmitted at what time and by which fieldbus (for example CAN) message. These settings cannot be modified later. Devices having a permanent configuration are often less expensive to purchase than devices of identical design which permit a configuration of the device during operation. The reasons therefore are the considerably simpler software structures and the transparent device implementation resulting therefrom. Field devices having an adjustable configuration have to be configured before being put into operation. All device parameters are defined using an appropriate configuration tool (ProCANopen, for example). The configuration thus obtained is then stored in the device. In a field device having an adjustable configuration, the network configuration created using the configuration tool is also stored in the field device provided that the devices support the “nonvolatile” storage (configuration data are not lost at a restart) of the configuration data. Alternatively, the device configuration can be stored in a configuration manager which performs the configuration of the configurable devices at a system startup. The configuration manager has to be defined already within the context of the network configuration. This approach permits the simple replacement of a defective device with a new device of identical design (simple maintenance of the system).
The disadvantage of the known device description files consists in that field devices can only be used in a known system. The use in a new or modified system requires an adjustment of the device description file and thus a replacement of the firmware of the field device.
To permit a higher flexibility at running time, it is known to use so-called “dummy objects”. These objects however constantly occupy resources which are not needed. Furthermore, dummy objects permit merely a limited flexibility and are confusing for the user.